The invention relates to an optical waveguide fiber designed for long distance, high bit rate telecommunications. In addition, the waveguide fiber is designed for long distance, high bit rate systems which use one or more optical amplifiers.
Telecommunication systems using high powered lasers, high data rate transmitters and receivers, and wavelength division multiplexing (WDM) technology require optical waveguide fiber having exceptionally low, but non-zero, total dispersion, and exceptionally low polarization mode dispersion (PMD). In addition, the waveguide fiber must have characteristics which essentially eliminate non-linear phenomena such as self phase modulation (SPM) and four wave mixing (FWM). The SPM can be limited by lowering power density. The FWM is controlled by operating in a wavelength range whereat dispersion is non-zero.
A further requirement is that the optical waveguide be compatible with long length systems incorporating optical amplifiers.
To provide an optical waveguide having the characteristics required for these sophisticated systems, a variety of refractive index profiles have been modelled and tested. The compound core design, discussed in U.S. Pat. No. 4,715,679, Bhagavatula, offers the flexibility to meet the new system requirements while maintaining the basic requirements such as low attenuation, narrow geometry tolerances, acceptable bending resistance, and high tensile strength. Furthermore, certain of the compound core designs are relatively easy to manufacture, thereby providing enhanced optical waveguide performance without prohibitive cost increases.
Translating these requirements into optical waveguide parameters, an optical waveguide suitable for high data rate and WDM systems and compatible with systems using optical amplifiers, are characterized by:
a mode field diameter sufficiently large to limit SPM; PA1 low residual stress and tight control of core and clad roundness and concentricity and coating uniformity to limit PMD; PA1 proper choice of coating modulus and glass transition temperature to limit externally induced stress birefringence; PA1 an absolute value of total dispersion in the WDM window sufficiently high to prevent FWM but low enough to limit the dispersion power penalty; and, PA1 an absolute value of total dispersion and zero dispersion wavelength compatible with operation in the optical amplifier gain peak wavelength region. PA1 The terms refractive index profile and index profile are used interchangeably. PA1 The radii of the regions of the core are defined in terms of the index of refraction. A particular region begins at the point where the refractive index characteristic of that region begins and ends at the last point where the refractive index is characteristic of that region. Radius will have this definition unless otherwise noted in the text. PA1 The initials PMD represent polarization mode dispersion. PA1 The initials WDM represent wavelength division multiplexing. PA1 The initials SPM represent self phase modulation, the phenomenon wherein portions of a signal above a specific power level travel at a different speed in the waveguide relative to portions of the signal below that power level. PA1 The initials FWM represent four wave mixing, the phenomenon wherein two or more signals in a waveguide interfere to produce signals of different frequencies. PA1 The term, % delta, represents a relative measure of refractive index defined by the equation, PA1 The term alpha profile refers to a refractive index profile, expressed in terms of % delta(r), which follows the equation, PA1 The profile volume is defined as integral from r=r.sub.i to r=r.sub.j of the quantity [(% delta(r))(r dr)]. PA1 the profile volume from the centerline to the cross over radius, the inner profile volume; PA1 by the profile volume from the cross over radius to the end of the core, the outer profile volume; and, PA1 by the ratio of outer to inner profile volume. The inner profile volume, outer profile volume and ratio of outer to inner profile volume are in the ranges of about 2.70 to 3.95 units, 1.10 to 7.20 units and 0.30 to 2.35, respectively. The units are % delta-microns.sup.2. The waveguide has a dual layer coating to preserve waveguide fiber strength and to substantially isolate the waveguide from external forces which can cause birefringence, thereby limiting polarization mode dispersion. The dual layer coating comprises an inner layer having an elastic modulus in the range of about 1.0 to 3.0 MPa and a glass transition temperature no greater than about -10.degree. C. Some coatings, such as a silicone, are known to have a glass transition temperature no greater than -180.degree. C. Acrylate coatings with glass transition temperature in the range of -30.degree. C. to -40.degree. C. are known. For the inner layer a lower transition temperature is better.